Water-extended polyester emulsions are used to make cured thermoset castings that are actually microcellular water-filled foams. The thermosets are useful as plaster replacements because of their high impact and tensile strengths. Water-extended polyester emulsions are ideal for reproducing intricate pieces of art because the emulsion flows easily into complex molds and gives cured products that resist breakage. Because water acts as a heat sink, thick articles with complicated shapes can be easily cast quickly and cured without overheating problems.
A water-in-resin emulsion is typically formed by adding water slowly to a polyester resin with high-shear stirring to form a continuous phase of polyester that contains, emulsified within the continuous phase, small water droplets (average diameter about 2-6 microns). The low-viscosity emulsion is poured into a mold and is cured with a catalyst system comprising a free-radical initiator and a transition-metal catalyst to produce the thermoset product. The thermoset product consists of a solid plastic continuous phase with water-filled cells as the discontinuous phase.
Emulsion stability is an important concern in preparing water-extended polyester emulsions. Even a properly made water-extended polyester emulsion typically remains stable for only a few hours. Because emulsion stabilities are so low, fabricators of water-extended polyester thermoset products normally purchase polyester resins and make their own emulsions. Thus, purveyors of the thermosets must normally purchase expensive, sophisticated mixing equipment and acquire expertise in formulating the emulsions from the polyester resins.
One approach to formulating more stable polyester emulsions is to use polyester resins of relatively high molecular weight. Coupling agents, such as diisocyanates, are sometimes used to link polyester chains together to improve emulsion stability (see, e.g., U.S. Pat. No. 4,077,931). Although polyesters with higher molecular weights can give stabler emulsions, they also generally have higher viscosities. High resin viscosity can be a disadvantage in making the thermosets because excess air entrapment can occur, resulting in products with unwanted voids.
Commercial water-extended polyesters give thermosets with tensile and flex properties that make them suitable for casting and decorative arts applications. Generally, formulators strike a balance between the level of properties needed and cost. Tensile and flex properties generally diminish steadily as the amount of water is increased. An improved emulsion would give thermosets having a high level of physical properties even at high water levels.
Improved water-extended polymer emulsions are needed. In particular, emulsions with improved stability relative to commercial polyester emulsions would offer substantial advantages. The availability of a stable emulsion would enable a manufacturer of water-extended thermoset products to eliminate the need to purchase expensive mixers and to acquire formulating expertise; instead, he or she could simply purchase the emulsion. Ideally, the emulsion could be stabilized without substantially increasing its viscosity.
Improved water-extended thermosets are also desirable. Preferred thermosets would have good physical properties over a wide range of water contents, particularly at high water levels.